Reference 3548 and 3716 are identical with the exception that the 3716 has subsidiary seconds and the 3548 has center seconds. In period advertisements this watch is called ?Impero?. The caliber 635 was produced between 1940 to 1951. This watch is illustrated in ?Rolex Collecting Wristwatches? by Osvaldo Patrizzi 2001 Edition pp. 166 167.